Download Part A: A model of DNA

Heinemann Biology Activity Manual
Activity 6.12 A model of DNA (Extension activity)
Skills and Outcomes: H9
Type of activity: First-hand investigation
Introduction
DNA (deoxyribonucleic acid) is the long molecule found in the chromosomes of cells that
carries the genetic or inherited information for that cell. The DNA molecule is made up of
repeating units or nucleotides. (See Heinemann Biology second edition, p. 61.) A sequence of
bases along a DNA strand is known as a gene. Each gene contains the information required to
make a protein or polypeptide for the cell. This information is first copied by the cell and then
used to synthesise the new molecule. The two processes involved in this are transcription and
translation. (See Heinemann Biology second edition, pp. 300–1)
This activity involves the construction of a simple model to illustrate the structure of DNA
and the process of protein synthesis. Other secondary sources can also be used as additional
and/or alternative sources of information.
Purpose
1
To develop a simple model of a section of DNA (part A).
2
To develop a simple model for protein synthesis (part B).
Materials
•
Heinemann Biology second edition text book
•
Other books, journals, Internet sites
•
Materials as listed separately in parts A and B
Part A: A model of DNA
Deoxyribonucleic acid (DNA), which is found mainly in the nucleus of cells, consists of a
double strand of building blocks called nucleotides. Each nucleotide is made up of three
molecules:
i
a cyclic sugar (deoxyribose)
ii
a phosphate group
iii
a nitrogen base.
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© Pearson Australia (a division of Pearson Australia Group Pty Ltd)
Heinemann Biology Activity Manual
Each strand of DNA is composed of linked sugar–phosphate groups. A base is attached to each
sugar in the strand. There are four types of bases in DNA:
Adenine—‘A’
Guanine—‘G’
Cytosine—‘C’
Thymine—‘T’
The bases along one strand link onto the bases of the second strand of DNA.
A only joins with T
G only joins with C
Materials
(per group)
•
Cord (to fit through the coils of pegs)—two 50 cm lengths
•
24 slide-on paper clips
•
Spring-type clothes pegs in 4 different colours—at least 6 of each colour
Procedure
For the purpose of building the model:
•
the cord will represent the sugar
•
the paper clip will represent the phosphate
•
the pegs will represent the bases.
1
Record the colour you have allocated to each base.
2
Pass the cord through the coils of twelve pegs in any arrangement.
3
Attach paper clips onto the cord, one in between each peg.
4
To each peg on your strand, clip on the complementary peg (remember: A with T, G with C).
5
String the pegs together with cord, and attach paper clips in between the pegs, to make
the second strand of a DNA molecule.
6
Sketch your DNA molecule. Label each part of the drawing with the chemical name of
the group it represents.
7
Twist the two strands around one another to make a double helix. The DNA molecule is
coiled in this manner.
Copyright
© Pearson Australia (a division of Pearson Australia Group Pty Ltd)
Heinemann Biology Activity Manual
Discussion
1
What symbol represents each of the following molecules? (Draw each one, and describe.)
i
sugar
ii
phosphate
iii
adenine
iv
thymine
v
cytosine
vi
guanine.
2
What are the components of a nucleotide?
3
DNA is often described as a ladder. What components make up:
4
i
the sides of the ladder
ii
the rungs of the ladder?
Name the complementary base pairs.
Part B: A simple model for protein synthesis
Ribonucleic acid (RNA), which is found throughout the cell, exists in a number of forms. Two
of these are known as messenger RNA (mRNA) and transfer RNA (tRNA). All RNA molecules
are single strands of nucleotides. They differ in composition from DNA in that:
•
the sugar is ribose, not deoxyribose.
•
the base uracil replaces the base thymine. Uracil is complementary to adenine.
Messenger RNA is made in the nucleus. At the start of the process of protein synthesis, mRNA
copies a series of bases from the DNA strand and takes this ‘message’ to a ribosome. tRNA
transfers amino acids to the ribosome from the cytoplasm. Each tRNA has an exposed triplet of
bases at one end and an amino acid specified by this triplet at the other end.
Proteins are manufactured at the ribosomes. A protein consists of a sequence of
particular amino acids. The sequence of bases in DNA determines the sequence of amino acids
in a protein. Each triplet of bases in DNA codes for one amino acid.
The DNA code in one gene is sent to the ribosomes in the form of mRNA (transcription)
and the instructions are followed, with the assistance of tRNA molecules, to make one protein
(translation).
Copyright
© Pearson Australia (a division of Pearson Australia Group Pty Ltd)
Heinemann Biology Activity Manual
Materials
(per group)
•
DNA model from part A
•
Pegs in a new colour, to represent uracil
•
Four 50 cm lengths of cord, to represent sugars
•
Paperclips, to represent phosphates
•
Materials to represent amino acids
Procedure
1
Record the colour you attributed to uracil.
2
Detach nine bases of the DNA model.
3
Using one side of the DNA only, make a complementary strand of mRNA. This will be
similar to the freed strand of the DNA you are not copying, except that:
•
U replaces T
•
ribose replaces deoxyribose.
To make the model of mRNA, find the nine complementary bases then join them up with
sugars and phosphates.
4
Use the mRNA to make your tRNA models: put complementary bases beside the first
three bases of the mRNA model. Use sugars and phosphates to build up the first tRNA
model.
5
Repeat step 4 with the two adjacent triplets of bases in the mRNA to make the second
and third tRNA models.
6
Attach an amino acid to each tRNA.
7
Spread out your mRNA sequence. Imagine it is connected to a ribosome.
8
Bring in your tRNA molecules to match the sequence in the mRNA.
9
Link amino acid 1 to amino acid 2 and detach them from their tRNAs.
10
Link amino acid 3 to the ‘protein’ you are making. Detach it from tRNA 3.
Copyright
© Pearson Australia (a division of Pearson Australia Group Pty Ltd)
Heinemann Biology Activity Manual
Discussion
1
Consider the exposed triplet of bases in a tRNA. What molecule has complementary
bases? What molecule has similar bases?
2
Explain the purpose of messenger RNA.
3
What must be present in ribosomes to link amino acids together?
4
In what ways did your model represent the process of protein synthesis?
Discussion/Conclusion (for parts A and B)
Produce a fully annotated sequence of diagrams to describe the processes of transcription and
translation in protein synthesis.
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© Pearson Australia (a division of Pearson Australia Group Pty Ltd)